Google said today it has shipped more than 5 million of its Cardboard VR headsets.

In a blog post, Google's new VR head Clay Bavor wrote that in the 19 months since Cardboard was first released, there have been more than 5 million units shipped, and that there have been more than 25 million installs of the Cardboard app from the Google Play store. Ten million of those installs occurred between October and December.

One thing that has helped get Cardboard into users' hands is that Google has allowed third parties to ship branded versions of Cardboard. Some of those include an R2-D2 model, a very popular New York Times model that shipped to millions of the paper's subscribers, and even a View-Master version.

There are more than 1,000 apps built for Cardboard available for Google Play, the company said, with four of the most popular five being games. Another is Vrse, a player from a leading VR content maker. The most popular Cardboard app is Chair in a Room.

Because YouTube's 360-degree videos are compatible with Cardboard, Google said that users have now viewed more than 350,000 hours of the videos on Cardboard. At the same time, users have taken more than 750,000 photos with Google's Cardboard Camera app.

Cardboard, of course, is just one type of VR player in a rapidly growing field. Last fall, Samsung released its mid-range Gear VR, which works with top-of-the-line Samsung smartphones, and this spring, Facebook-owned Oculus will release its high-end Rift, HTC will launch its high-end Vive, and Sony will put out its PlayStation VR. Analysts predict that by 2020, the entire virtual reality ecosystem will generate $30 billion in annual revenue, while VR content alone is expected to be worth $5.4 billion a year by 2025.

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